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Two Component Polyurethane Coating Systems

2K Coatings Offer Hardness and Chemical Resistance

The most frequently recommended products for two-component polyurethane coatings, also know as 2K coatings, applications are Desmodur® N polyisocyanates and Desmophen® polyols. In addition, for some applications the Desmodur and Mondur® aromatic polyisocyanates may also be appropriate.

applying a two component coating

You can find this information and accompanying illustrations in the Bayer publication, “The Chemistry of Polyurethane Coatings” by clicking here.

 

Calculations for Two Component Coatings

When preparing two component polyurethane coating formulations, the rules for chemical equations are followed. In theory, a maximum molecular weight is reached and those properties associated with molecular weight are optimized when one equivalent of isocyanate reacts with one equivalent of hydroxyl, that is, when the ratio of NCO to OH is 1.0 / 1.0. In practice, a small excess of isocyanate, about 5-10%, is often used to allow for the likely destruction of isocyanate by water contained in solvents and pigments, so that the NCO to OH ration is maintained at 1.0 / 1.0. It is sometimes desirable to vary the NCO to OH ratio from about 0.9 / 1.0 to about 1.2 / 1.0 in order to modify the properties of the coating. In any event, it is important that the NCO to OH ratio be controlled.

In order to achieve an NCO to OH ratio of 1.0 / 1.0, the weight of one equivalent of the supplied form of the isocyanate is reacted with one equivalent of the supplied form of the polyol. Usually the product literature will list the equivalent weights, but if not, then the polyisocyanate literature will give the %NCO and the polyol literature will give the %OH or the OH number. The NCO equivalent weight can be calculated from the %NCO, and the OH equivalent weight from either the %OH or the OH number. These equations can be combined to calculate the weight ratios of ingredients for any NCO to OH ratio. Basic algebra is all that is necessary for these calculations.

 

Properties of Two Component Coatings

The properties of a two-component polyurethane coating depend on certain factors, such as the degree of branching of the reaction partners, the content of the reactive groups themselves, and the physical state of the raw materials.

It is possible to change the film properties by using the same polyisocyanate with different polyols while maintaining a constant NCO to OH ratio. Generally speaking, the higher the hydroxyl content of the particular reaction partner, the harder and more chemical resistant the films will be. A low hydroxyl content normally gives softer and more flexible films.

A further possibility for modifying the film properties is to use the same polyol and vary the quantity of the polyisocyanate. By “undercrosslinking,” which means using an NCO to OH ratio less than 1.0 / 1.0, the polyurethane film generally becomes softer and perhaps more flexible, less weather resistant, and less resistant to solvent and chemicals. On the other hand, by “overcrosslinking,” using an NCO to OH ratio greater than 1.0 / 1.0, the resultant films tend to be harder and more chemical resistant. Another possibility for changing the properties of a coating is to use a blend of polyols or polyisocyanates. For instance, a rigid, brittle acrylic urethane coating could be made less brittle and more flexible by blending in a softer, less-branched polyester. Also, a slower drying polyurethane coating could be made to dry more quickly by blending some faster reacting or faster drying polyisocyanate with the original. For example, blends of Desmodur Z and Desmodur N polyisocyanates are used to take advantage of the faster physical dry of Desmodur Z and the higher chemical crosslinking of the Desmodur N polyisocyanates.

 

Curing of Two Component Coating Systems

The curing of two-component polyurethane coating systems can be carried out at room temperature. The drying times can vary considerably depending on the types of polyols and polyisocyanates used. In general, systems based on the following polyisocyanates dry at room temperature in these approximate relative times:

Desmodur IL
1
Desmodur HL
2
Desmodur L
6
Desmodur N + catalyst
6
Desmodur N
30

Elevated temperatures are often used for forced drying in production line coatings. The highly reactive Desmodur IL and Desmodur HL polyisocyanates have particular importance for baked finishes as they allow extremely short drying times. Coatings based on Desmodur L or Desmodur N polyisocyanates can also be cured at elevated temperatures to reduce the cure time.

Desmodur N aliphatic polyisocyanates have low reactivity by nature, and longer drying times can be expected. The incorporation of 0.005% dibutyltin dilaurate, 0.2% zinc octoate, or up to 0.5% Desmorapid® PP catalyst based on solid binder reduces the dry time to practical levels.

While coatings based on Desmodur L polyisocyanate normally do not require a catalyst, the addition of an accelerator may be advisable for certain applications. Metal-based catalysts have less of an effect on Desmodur L polyisocyanates than do tertiary amines such as Desmorapid PP catalyst, which is normally used in the range of 0.05-0.2% based on solid binder.

Coatings based on Desmodur IL or Desmodur HL polyisocyanates are normally used without a reaction accelerator.

 

Two Component Coating System Application

 

The chemical curing mechanism of two-component polyurethane coatings makes it necessary to consider the following points:

  1. Both components must be mixed together thoroughly.
  2. The reaction which begins immediately upon mixing results in a gradual to rapid viscosity increase.
  3. The increase in viscosity eventually leads to a gelation of the paint.

The usable potlife depends on the following factors:

  • Binder concentration and coating solids level
  • NCO/OH ratio
  • Polyol and polyisocyanate used
  • Catalyst type and concentration
  • Temperature of the coating mixture
  • Choice and quality of the solvents
  • Possible effects of additives, pigments, or extenders

If these factors are taken into account, the potlife of conventional two-component polyurethanes can be set to be one work shift. In principle, this means any application method can be employed. Faster curing coatings require an adjustment in the application method.

Two component polyurethane coatings can be applied by a variety of methods. Application by air, airless, or air-assisted airless spray, electrostatic spray, brush, roller coating, and curtain coating is possible. Careful formulation is required to maximize the application characteristics of each formulation for its end use.

 

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